1. Field of the Invention
The present invention relates to a domain wall displacement readout type magneto-optical disc and a manufacturing method thereof and, more in particular, to a method for annealing anneal tracks that exist at both sides of an information recording track.
2. Related Background Art
As a rewritable high density recording system, there is a system available wherein, by using thermal energy of a semiconductor laser, a magnetic domain is written in a magnetic thin film to record information and, by using a magneto-optical effect, this information is read. Further, in recent years, there has been an increasing demand for further increasing the recording density of the magneto-optical disc of this system so as to make it as a large-capacity recording medium. By the way, a line recording density of the magneto-optical disc and the like largely depends on a laser wave length λ of a reproduction optical system and a numerical aperture NA of an objective lens. In other words, when the laser wave length λ of the reproduction optical system and the numerical aperture NA of the objective lens are decided, the diameter of a beam waist is decided and, therefore, a spatial frequency at the time of reproducing a recorded domain has a detectable limit only at about 2 NA/λ.
Accordingly, in order to realize high density by the conventional magneto-optical disc, it is necessary to shorten the laser wave length of the reproduction optical system and enlarge the NA of the objective lens. However, there is a limit to improvement of the laser wave length and the numerical aperture of the objective lens. For this reason, a technology to think out a constitution of the recording medium and a reading method and improve recording density is being developed.
For example, in Japanese Patent Application Laid-Open No. 06-290496, the magneto-optical disc and its manufacturing method are disclosed, the disc using a perpendicular magnetic anisotropy multi-layer film having at least s domain wall displacement layer magnetically linked, a switching layer and a memory layer.
This method uses an ingenious mechanism, wherein, at the time of reproduction, a thermal gradient to be generated by irradiation of an optical beam is used and the domain wall of a recorded mark of the domain wall displacement layer is displaced without changing recorded data in the memory layer, and the domain wall displacement layer is magnetized so that a part of an optical beam spot area is uniformly magnetized and a change of the polarization plane of the reflected light of the optical beam is detected, thereby reproducing a recorded domain of the cycle below a diffraction limit.
By using this reproduction system, a reproduction signal becomes rectangular (FIG. 11D), and it is possible to reproduce the recorded mark of the cycle below the diffraction limit of a light without lowering the reproduction signal amplitude by depending on an optical resolving power, and the magneto-optical disc capable of considerably improving the recording density and a transfer velocity becomes possible.
Note that, in this type of magneto-optical disc, in order to utilize the temperature gradient by irradiation of the light beam so as to easily cause the displacement of the domain wall of recorded mark of the domain wall displacement layer, a laser beam of high power is irradiated at the portion of adjacent two pieces of the anneal tracks (guide grooves) which make the information recording track of the magneto-optical disc exist between them, and a magnetic layer of the anneal track (guide groove) is annealed at high temperature and subjected to an annealing process which degenerates a magnetic layer of the portion of the anneal track (guide groove). By this annealing process, a switched connection between the information recording tracks is disconnected and the domain wall is not formed along the side portion of the information domain track of the recorded mark. As a result, the action of a domain wall coercivity is reduced, and more stabilized displacement of the domain wall becomes possible. This annealing process can obtain a good reproduction signal.
The reproducing action of the domain wall replacement type magneto-optical disc will be described by using FIGS. 11A to 11D. Here will be dealt with the constitutions of three layers: a memory layer which governs the storing of the recorded mark; the domain wall displacement layer where the domain wall displaces and directly contributes to the reproduction signal; and a switching layer which switches a link status between the memory layer and the domain wall displacement layer.
FIG. 11A is a typical view which shows a magnetic domain reproducing state. A thick line 111 shows a domain wall of the domain wall displacement layer, and a narrow line 112 shows the domain wall of the memory layer only. FIG. 11B shows a state graph of a recording film, FIG. 11C a temperature state graph of a medium and FIG. 11D the reproduction signal. Note that the two pieces of the anneal tracks (guide grooves) which make the information recording track exist between them, as described above, subjected to the annealing process where a magnetic layer is degenerated by irradiation of high powered laser beam. At the time of reproduction, the anneal track is heated until a Ts temperature condition (FIG. 11A) where the domain wall of the domain wall displacement layer of a domain wall displacement medium is displaced by irradiation of a light beam 116. Here, the Ts is the Curie point of the matter which constitutes the switching layer, and the switching layer 22 (FIG. 11B) is in a link state with the memory layer 21 and the domain wall displacement layer 23 by the switched connection in a low temperature area. When the magneto-optical disc displaces in the direction shown by an arrow mark 114 and is heated more than the Ts temperature by irradiation of the light beam, the link between the domain wall displacement layer and the memory layer is put into a disconnected state (inside of a Ts constant temperature line shown by the Ts of FIG. 11A. For this reason, as soon as the domain wall of the recorded mark arrives at this Ts temperature area, an effect of the annealing process (annealing process portion by laser is shown by reference numeral 113 in FIGS. 11A to 11D) of the two pieces of the anneal tracks (guide grooves) adjacent to the information recording track also takes place, and the domain wall of the domain wall displacement layer instantaneously displaces to the position where the domain wall can stably exist energy-wise in relation to the temperature gradient of the domain wall displacement layer, that is, to the direction of an arrow mark 115 so that the domain wall can cross the information recording track at the highest temperature of the line density direction of the temperature rise by the light beam irradiation. In this way, a large portion of magnetic state of an area S which is covered by the reproduction light beam becomes the same and, therefore, in the usual light beam reproduction principle, even if it is a minute recorded mark which is not possible to reproduce, a reproduction signal nearly in a rectangular shape as shown in the drawing can be obtained.